Ubuntu 19.10 - Refcount Underflow and Type Confusion in shiftfs

Ubuntu 19.10 - Refcount Underflow and Type Confusion in shiftfs Tested on Ubuntu 19.10, kernel "5.3.0-19-generic 20-Ubuntu". Ubuntu ships a filesystem "shiftfs" in fs/shiftfs.c in the kernel tree that doesn't exist upstream. This filesystem can be mounted from user namespaces, meaning that this i...